Schools know how many people they are going to interview, usually 20 to 30. Most applications are in by Thanksgiving. If schools are calling early, they are typically only filling 10 to 12 spots with their absolute top candidates. They will reserve a few spots for applications up to the deadline. If they don't call their top candidates now, the candidates' calendar might be booked when they do call.

University of Guelph has a deadline in mid December. Do they have early calls?

List the universities that have already called. I know of three
University of Guelph
Louisiana State University
Imperial College